1. Field of the Invention
The present invention relates to the field of fiberoptic transmission and in particular to wavelength-division multiplex fiberoptic transmission systems.
2. Description of the Prior Art
The index profile of optical fibers is generally qualified as a function of the shape of the graph of the function relating the refractive index to the radius of the fiber. It is conventional to plot on the abscissa axis the distance r to the center of the fiber and on the ordinate axis the difference between the refractive index at the radius r and the refractive index of the cladding of the fiber. The terms xe2x80x9cstepxe2x80x9d, xe2x80x9ctrapeziumxe2x80x9d and xe2x80x9ctrianglexe2x80x9d index profiles are therefore used for graphs which are respectively step-shaped, trapezium-shaped and triangular. The curves are generally representative of the theoretical or setpoint profile of the fiber and fiber fabrication constraints can lead to a significantly different profile.
In new high bit rate wavelength-division multiplexed transmission networks it is advantageous to control chromatic dispersion, especially for bit rates of 10 Gbit/s and above; the objective is to obtain substantially zero cumulative chromatic compensation over the link for all wavelength values of the multiplex, in order to limit broadening of the pulses. A cumulative dispersion value of a few hundred ps/nm is acceptable. It is also beneficial to avoid zero values of chromatic dispersion in the vicinity of the wavelengths used in the system, for which the non-linear effects are greater. Finally, it is also beneficial to limit the chromatic dispersion slope over the range of the multiplex to prevent or limit distortion between the channels of the multiplex.
Stepped index fibers are conventionally used as line fibers in fiberoptic transmission systems. ASMF 200 stepped index monomode fiber has a chromatic dispersion cancellation wavelength xcex0 from 1 300 nm to 1 320 nm and an absolute chromatic dispersion less than 3.5 ps/(nm.km) in a range from 1 285 nm to 1 330 nm and equal to 18 ps/(nm.km) at 1 550 nm. The chromatic dispersion slope at 1 550 nm is in the order of 0.05 ps/(nm2.km).
A stepped index fiber having a core whose index is 5xc3x9710xe2x88x923 greater than the index of the cladding typically has a chromatic dispersion cancellation wavelength xcex0 around 1 310 nm. It can be obtained by doping the silica with germanium, which increases the chromatic dispersion cancellation wavelength xcex0. There is zero chromatic dispersion in the silica for a wavelength xcex0 of 1 285 nm; doping with germanium increases this value to 1 310 nm.
It is well known in the art that non-linear effects decrease as the effective area of the fiber increases. The article by M. Kato et al., xe2x80x9cA new design for dispersion shifted fiber with an effective core area larger than 100 xcexcm2 and good bending characteristicsxe2x80x9d, ThK2, OFC""98 Technical Digest, explains that non-linear effects in the fibers could become the dominant capacity and transmission distance limitations for high-capacity long-haul amplified transmission systems. The document indicates that one solution is to increase the effective area of the fibers, which enables a higher power and a greater distance between repeaters to be obtained. The document proposes a fiber having a coaxial profile, surrounded by a pedestal, with an effective area of 146 xcexcm2 and a chromatic dispersion cancellation wavelength xcex0 equal to 1 500 nm. The chromatic dispersion at 1 550 nm is low and the dispersion slope at this wavelength is equal to 0.09 ps/(nm2.km).
The article by D. Bayart and S. Gauchard, xe2x80x9c50 GHz channel spacing analysis in Nxc3x972.5 Gbit/s systemsxe2x80x9d, OFC""98 Technical Digest WD2, describes a wavelength-division multiplex transmission system with 32 to 80 channels or more having a unit bit rate of 2.5 Gbit/s. The channels lie within a range of wavelengths from 1 530 nm to 1 560 nm with a spacing of 50 GHz, i.e. 0.4 nm between adjacent channels.
In wavelength-division multiplex transmission systems known in the art, the signals are transmitted within this range of wavelengths but not within the range of wavelengths around 1 300 nm, referred to as the second window.
The invention proposes to use not only the transmission window around 1 550 nm but also the transmission window around 1 300 nm in a wavelength-division multiplex transmission system. To limit non-linear effects, the invention proposes a stepped index fiber whose chromatic dispersion is not less than 2 ps/(nm.km) for wavelengths above 1 280 nm; compared to a stepped index fiber known in the art, this can be achieved by reducing the chromatic dispersion cancellation wavelength xcex0.
To be more precise, the invention proposes a stepped index optical fiber having a chromatic dispersion cancellation wavelength not greater than 1 280 nm.
In one embodiment of the invention, the fiber has a chromatic dispersion cancellation wavelength not greater than 1 250 nm.
The fiber preferably has a chromatic dispersion not less than 2 ps/(nm.km) for a wavelength not less than 1 280 nm.
The fiber advantageously has a core whose index is substantially constant and a cladding whose index is less than the index of the core.
In one embodiment of the invention the fiber has a core whose index is less than the index of silica.
In one embodiment of the invention the fiber has a core and a cladding doped with fluorine or boron.
The core of the fiber preferably has a radius of not less than 4.5 xcexcm.
The invention also relates to a wavelength-division multiplex transmission system using fiber of the above kind as line fiber.
Other features and advantages of the invention will become apparent on reading the following description of embodiments of the invention which is given by way of example only and with reference to the accompanying drawings.